Page 112 - Troubleshooting Analog Circuits

Pease’s Principle

For many years, I’ve been cautioning people: oscillation may disappear with heavier capaci-
If you have a regulator or an amplifier circuit tive loads.
and it oscillates, do not just add resistors or The adjacent figure does not show a rec-
capacitors until the oscillation stops. If you ommended value for some of the parts. If you
do, the oscillation may go away for a while, are testing a SA regulator, you may want a
but after it lulls you into complacency, it will load resistor as low as an ohm or two. If you
come back like the proverbial alligator and are evaluating a low-power amplifier or a
chomp on your ankle and cause you a great micropower reference, a value of a megohm
deal of misery. may be reasonable, both for the load resistor
Instead. when you think you have designed and for the resistor from the square-wave
and installed a good fix for the oscillation, generator. Thinking is recommended. Heck,
BANG on the output with square waves of thinking is required.
various amplitudes, frequencies, and amounts When you bang a device’s output and that
of load current. One of the easiest ways to output rings with a high Q, you know your
perform this test is to connect a square-wave “fix” doesn’t have much margin. When the
generator to your circuit through a couple output just goes “flump” and doesn’t even
hundred ohms in series with a 0.2-pF ceramic ring or overshoot appreciably, you know
disc capacitor. Connect the generator to the your damping is effective and has a large
scope, SO you can trigger on the square-wave safety margin. Good! Now, take a hair dryer
signal. Also, apply an adjustable DC load and get the circuit good and warm. Make sure
that’s capable of exercising the device’s that the damping is still pretty well behaved
output over its full rated output-current and that the output doesn’t begin to ring or
range or, in the case of an op amp, over its sing when you heat the capacitor or power
entire rated range for output voltage and transistor or control IC or anything.
current. I don’t mean to imply that you shouldn’t do
To test an op amp, try various capacitive a full analysis of AC loop stability. But the
loads to make sure it can drive the worst approach I have outlined here can give you
case you expect it to encounter. For some pretty good confidence in about five minutes
emitter-follower output stages, the worst that your circuit will (or won’t) pass a full set
case may be around IO to 50 pF. The of exhaustive tests.